1. Field of the Invention
The present invention relates to a circuit board, particularly relates to a circuit board wherein electronic parts and electrical components are mounted on the surface of a substrate such as a semiconductor chip.
2. Description of the Related Art
Currently, a circuit board is used for various electric equipment and information equipment. In a conventional circuit board, electronic parts are mounted on the top surface or the bottom surface of a substrate. As recent electric equipment and information equipment are miniaturized and are provided with high functions, technique for further integrating mounted electronic parts, forming a pattern of a few layers inside a circuit board and forming a circuit wherein these patterns are mutually connected has been also enhanced.
Such technique has been remarkably developed for a circuit board particularly used for a computer, however, as a digital signal is conducted in a circuit board used for this technical field, an electromagnetic wave is emitted from the circuit board as EMI noise. As EMI noise has an effect upon another circuit provided to the same electric equipment and information equipment or another electric equipment and information equipment, it is required to be reduced.
To prevent EMI noise from leaking outside the chassis of electric equipment and information equipment, a conductive sheet was arranged inside the chassis of electric equipment and the information equipment or the chassis was made of conductive material as a major conventional technique. Also, for another technique to reduce EMI noise in the electric equipment and the information equipment on which many circuit boards are mounted, a shield plate for shielding an electromagnetic wave was provided between circuit boards, or the surface of a circuit board is shielded in a state that electronic parts are mounted.
However, it has been difficult to reduce EMI emitted noise as the performance of electric equipment and information equipment is enhanced. Emitted EMI noise cannot be reduced by only applying the shield to the chassis of electric equipment and information equipment. Therefore, a method of directly reducing emitted EMI noise caused from a circuit board is required.
The object of the invention is to provide a circuit board wherein EMI noise emitted from the circuit board is reduced.
Another object of the invention is to provide a circuit board that can reduce noise mixed from an external device.
According to one aspect of the present invention, a substrate is provided which includes: pads which are provided on the surface of said substrate; and surface layers which are kept to the ground potential and cover the surface of said substrate except said pads and their peripheral.
According to another aspect of the present invention, a substrate is provided which includes: a part of circuit which is provided on the surface of said substrate; and a surface layers which are kept to the ground potential and cover the surface of said substrate except said part of circuit and its peripheral.